Vertical distributions of macroplankton and micronekton in the Ligurian and Tyrrhenian Seas (northwestern Mediterranean)

Vertical distributions and diel migrations at nineteen stations located in the Ligurian and Tyrrhenian Seas (northwestem Mediterranean) are described in detail and compared for the main species of micronekton and macroplankton. Sampling was performed in April 1994 in the 0-700 m water column in most cases, down to 350, 450 or 550 m for the coastal or relatively shallow sites. Most of the twenty species showed similar distributions throughout the sampled area, either non-diel-migrating distributions such as the euphausiid Stylocheiron longicorne and the fish Cyclothone braueri, or clear diel-migrant behaviour (e.g. the siphonophore Chelophyes appendiculata and the euphausiid Euphausia krohni). Four species, the siphonophore Lensia conoidea, the pteropods Cavolinia inflexa and Clio pyramidata and the salp Salpa fusiformis, showed heterogeneity between distributions, with different preferential daytime depths. For example, moderate or extensive migration and even slight reverse migration were observed in Lensia conoidea, depending on the station sampled. The differences in vertical distributions did not appear to be related to hydrological conditions (temperature, salinity); but rather to chlorophyll content land ambient light) or bottom depth of the station, or to mating behaviour of the species.